switch to osmarender (Box in the upper right corner), there is no disturbing label …
In the display of Mapnik, there is no other solution at the moment. You could add a feature request in the OSM-trac-system that mapnik should not place a ref-label on very short segments of a street
Please create a ticket for this, if it isn’t already there. We’ve had one for refs on tunnels already, and I was meaning to add the same fix for bridges, but I forgot. Having a ticket will serve as a reminder.